Helicopters are equipped with many sensors that advise the pilot of the condition of various onboard systems. There are normally open sensors, which close the circuit when the sensor is activated, and normally closed sensors, which open the circuit when activated. The sensors are connected to indicators such as warning lights on the pilot's instrument panel. The sensor is typically located remote from the indicator. Therefore, the electrical connections typically pass through several harnesses, junction boxes, terminal boards, etc.
An example of a normally open sensor is a chip detector. A chip detector is used to monitor the health or air worthiness of a helicopter's transmission or gear box, which is a vital piece of equipment. The presence of a significant number of metal chips in the transmission fluid usually indicates mechanical problems with the transmission. The chip detector is partially immersed in the transmission fluid so as to be exposed to the metal chips circulating inside of the fluid. The chip detector is provided with a magnet so as to attract and retain the metal chips. The presence or absence of metal chips captured by the chip detector is indicated both visually and electrically. The electrical indication is provided by a warning light on the instrument panel. If metal chips accumulate during flight, the warning light is illuminated and the pilot can safely land the helicopter, before the rotors lock up.
The visual indication is provided between flights by a ground mechanic. The mechanic physically removes the chip detector from the transmission, visually inspects the collection area on the chip detector for metal chips, and then reinstalls the chip detector into the transmission. A visual inspection of the chip detector is required after the helicopter is flown for a specified number of hours. (In Canada, the chip detector is required to be visually inspected every day.)
Several problems can and have arisen due to the frequent removal and installation of chip detectors. Because the chip detector is in contact with transmission fluid, failure to properly reinstall the chip detector could result in a loss of fluid during flight. In fact, this very problem occurred in a helicopter flying over the Gulf of Mexico. The loss of transmission fluid during flight resulted in a forced landing of the helicopter on the water. One of the flotation devices on the helicopter failed, resulting in the helicopter flipping over and sinking.
Thus, with the required frequent handling of the chip detector component of the transmission, the possibility for loss of life or aircraft due to human error is significant. The electrical indication circuit provides no clue as to improper installation of an open circuit sensor such as a chip detector. What is needed is a system for detecting the improper installation of a chip detector.
Another problem caused by frequent handling of the chip detector is broken wires. Wires lead from the chip detector to the warning light in the cockpit instrument panel. These wires can be easily broken as the chip detector is handled during the visual inspection process. A broken wire results in the disablement of the electric circuit. In the prior art, there is Berder, et al., U.S. Pat. No. 5,045,840, owned by the assignee of the present invention. Berrier, et al. provides a continuity sensor that can be installed across an open circuit device such as a chip detector. Upon the application of power to the circuit in the cockpit, the continuity sensor temporarily closes the circuit to illuminate the warning light. If the warning light illuminates, the interconnections leading from the warning light to the chip detector are in working order. However, if the warning light fails to illuminate, then the chip detector circuit is inoperable.
The Berrier, et al. continuity sensor has proven to be a noteworthy and much needed device. Before the Berder, et al. continuity sensor, prior art electrical sensing circuits with normally open sensors were vulnerable to open circuit faults. With the Berrier continuity sensor, such open circuit faults can be identified and corrected.
It is desired to supplement the Berrier, et al. continuity sensor to, as mentioned above, detect if a chip detector has been improperly installed. In addition, it is desired to provide a system to monitor the continuity of wires leading to the chip detector on a continuous basis. Furtherstill, it is desired to provide a system to monitor the electrical status of the chip detector or other open circuit sensor so as to detect degradation of the contacts.
By virtue of its magnetic field, a chip detector installed in a transmission attracts chips of all sizes. Much of the metallic chips that are attracted to and retained by the chip detector are referred to as fuzz by the aircraft industry. This fuzz is produced by normal wear of components and represents no danger to the helicopter or aircraft. New transmissions and engines in particular produce relatively large amounts of fuzz during their break-in periods. This fuzz builds up in the chip detector, causing a short across the contacts of the chip detector. Thus, the fuzz is detected by the chip detector in the same manner as are larger chips.
Ideally, the chip detector would only detect the presence of large chips. These large chips indicate that the piece of equipment that is being monitored has internal components that are failing and therefore a catastrophic failure of the equipment is possibly imminent.
The problem then is how to distinguish between the relatively harmless fuzz and the larger size chips, which indicate a problem with the equipment being monitored. One way is to pull the chip detector out of its hole and visually inspect it to determine the size of the chips. But, as discussed above, this causes more problems (in the form of a broken wire or potential loss of transmission fluid if the chip detector is incorrectly reinstalled) than it solves. Also, visual inspections are unwise during flight.
There is a real need for a device that allows a pilot, during flight, to verify if the chip detector has detected large chips or just nuisance fuzz. Too many false alarms caused by nuisance fuzz degrade the effectiveness of the chip detector system, as a pilot is more likely to attribute a chip indication to just another false alarm.
In the prior art, there is Tauber, U.S. Pat. No. 4,070,660, which shows an electrical circuit that burns off the fuzz. A capacitor is connected across the chip detector contacts. When no chips are present in the chip detector, the capacitor charges to a voltage. When a chip enters the chip detector, the capacitor discharges through the chip. The idea is that the energy provided by the discharging capacitor will heat and burn away the unwanted fuzz, while leaving the larger chips, which require detection, in the chip detector. The use of electrical current to burn away small sized chips relies on the phenomenon of resistive heating. As current is passed through the chip, the resistance in the chip causes heating. It is hoped that the temperature increases to the point of melting or burning the chip.
The problem of the Tauber fuzz burner is its unreliability. This is due to the nature of the energy provided by a discharging capacitor. A capacitor discharges exponentially, with the peak discharge current through the chip occurring at the beginning of the discharge. Thus, the peak energy is delivered to the chip at the beginning of the discharge. In practice, this produces instantaneous power at the points of contact between the chip and the chip detector, resulting in welding the chip to the contacts. Thus, instead of burning fuzz away from the contacts of a chip detector, the Tauber device does just the opposite.
Furthermore, the contacts of the chip detector are immersed in oil circulating through the transmission. This immersion is necessary, as it exposes the contacts to the chips. However, the oil acts as a heat sink around the chips. This requires more energy to burn away a particular chip than if the chip was simply surrounded by air. The prior art capacitor circuit is often unable to deliver sufficient energy to burn away fuzz immersed in the oil.